1. Field of the Invention
The present invention relates to a method for preparing continuous, seamless blown thermoplastic film tubing by melt extruding a thermoplastic resin. In particular, it relates to apparatus and method for controlling the air pressure in the extruded film bubble as it advances from the die to a pair of collapsing nip rollers.
2. Description of the Prior Art
It is well known in the plastic art to continuously melt extrude thermoplastics through an annular orifice, apply internal fluid pressure to the extruded tube to support and expand the tube and reduce the wall thickness thereof to appropriate dimensions while cooling and solidifying the extruded thermoplastic. Collapsing pinch rollers, which draw the transversely expanded bubble in the longitudinal or machine direction, also served to flatten the formed tube into a double thickness. The flattened tube may be wound into a cylindrical roll for storage and subsequent use as a tube, or the tubing may be slit to form a single thickness sheet of double width wound into one roll, or two single thickness sheets wound into two separate rolls.
The fluid under pressure, usually air, which is used to inflate the extruded tubing is generally forced into the tubular film through an orifice within the tubular film (e.g. from the central mandrel portion of the die) under a substantially constant pressure which is sufficient to effect a desired radial expansion, and thus molecular orientation, of the successive portions of the tubular film after they leave the die and while they are being externally cooled to an orientation temperature exteriorly of the die. This external cooling of the extruded tube may be effected utilizing conventional cooling rings, mounted coaxially around the outer circumference of the tube. The rings supply cool air, under pressure, which impinges upon the surface of the advancing, inflated tube. Alternative techniques for imposing a desired shape on an extruded deformable thermoplastic tube are known in the prior art. For example, U.S. Pat. No. 3,551,540, the disclosure of which is incorporated herein by reference, discloses a method and apparatus for shape imposition. Finally, the tubing is collapsed by being passed through a pair of nip rollers. The bubble of air, under pressure, within the film tube may be thought of as an internal, orientation mandrel whereby film is being continuously expanded and drawn over the air-mandrel by the driven nip rollers which collapse the tube.
In a conventional tubular extrusion process, the air within the bubble is controlled in volume as necessary to establish the required bubble diameter, and the pressure is normally not controlled in any way. However, in a process in which the shape of the bubble is imposed by an external means, the bubble diameter and thus the volume of internal air is established by the shaping device, and it is necessary to carefully control the pressure of the air within the bubble, in order to maintain the extruded tubing in proximity to the shaping device, as well as to keep the bubble rigid while it is being collapsed.
As hereinbefore noted, during the tubular extrusion operation the air pressure within the extruded bubble remains relatively constant. Accordingly, if the bubble diameter becomes larger such as during start-up, for example, or due to a process change, the volumetric capacity of the bubble becomes larger, and in order to maintain the required pressure it is necessary to add air to the inside of the bubble. Conversely, if the diameter of the bubble becomes smaller, for example during shut-down operations or due to a process change, it is necessary to remove air from the bubble or allow it to escape to the atmosphere.